Contactless electrical thermostat employing a bimetallic strip

ABSTRACT

A thermostat employing a bimetallic strip carrying a permanent magnet and a saturable magnetic core positioned at an adjacent fixed location is disclosed. The magnetic core has a primary winding that is supplied with electrical pulses and a secondary winding which is coupled to an output circuit, preferably including a Schmitt trigger circuit. When the bimetallic strip is bent due to the environmental temperature so that the permanent magnet is closely magnetically coupled to the magnetic core, the core approaches saturation and the pulse output signal decreases in magnitude. The output circuit is constructed to have electrical hysteresis so that the &#39;&#39;&#39;&#39;turn on&#39;&#39;&#39;&#39; and &#39;&#39;&#39;&#39;turn off&#39;&#39;&#39;&#39; levels of the circuit are different.

United States Patent [1 1 Camillo I 1 CONTACTLESS ELECTRICAL THERMOSTAT EMPLOYING A BIMETALLIC STRIP [75] Inventor: Charles Carl Camillo, Western Springs, 111.

[73] Assignee: Illinois Tool Works, Inc., Chicago, 111.

[22] Filed: Nov. 16, 1973 [21] Appl. No.: 416,490

[52] US. Cl 307/117, 337/1, 323/90 [51] Int. Cl. IIOIh 35/00 [58] Field of Search 1. 307/117, 116; 337/1, 3,

[56] References Cited I UNITED STATES PATENTS 7 3,305,770 2/1967 Hulls 323/90 X 3,344,850 10/1967 De Forest 3.471.718 10/1969 Weisz 307/290 OTHER PU BLlCATlONS Book: The Way Things Work; pages 20-21, published 1. Mar. 4, 1975 by Simon and Shuster, Rockefeller Center, 630 Fifth Ave., N.Y., N.Y., Copyright 1967.

George Allen & Unwin Ltd.

Primary E.\'aminer-Robert K. Schaefer Assistant Examiner-M. Ginsburg [57] ABSTRACT A thermostat employing a bimetallic strip carrying a permanent magnet and a saturable magnetic core positioned at an adjacent fixed location is disclosed. The magnetic core has a primary winding that is supplied with electrical pulses and a secondary winding which is coupled to an output circuit, preferably including a 'Schmitt trigger circuit. When the bimetallic strip is bent due to the environmental temperature so that the permanent magnet is closely magnetically coupled to the magnetic core, the core approaches saturation and the pulse output signal decreases in magnitude. The I output circuit is constructed to have electrical hysteresis so that the turn on" and turn off" levels of the circuit are different.

v 12 Claims, 4 Drawing Figures POWER SUPPLY SCHMlTT TRIGGER CIRCUIT RETRlGGElZABLE MULTIVI BRA T012 PATENTEDHAR 191s MULTIVlEiZATOR SCHMITT TRIGGER CIQCUIT.

Fig. 4

CONTACTLESS ELECTRICAL THERMOSTAT EMPLOYING A BIMETALLIC STRIP BACKGROUND OF THE INVENTION Bimetallic strips have been advantageously used in thermostat applications for many years. However, in the conventional bimetallic thermostat the bimetallic member either has a contact portion, or it carries a separate contact element, which contacts a fixed contact element. As is well known, mechanical contacts for electrical circuits cause many problems since they be-.

come pitted and corroded, which limits their life span; and they are subject to contact bounce making for unreliable operation. In addition, the moving contact fixed to the bimetallic strip must be secured to an electrical wire which becomes another source of potential failure of the thermostat.

It is an object of the present invention to provide a contactless thermostat of the bimetallic type in which a permanent magnet is secured to a bimetallic strip so as to interact with a saturable magnetic core which has a primary winding coupled to a sourceof input pulses and a secondary winding which is coupled to output circuitry which preferably is constructed to provide electrical hysteresis.

It is a further object of the present invention to provide an electrical thermostat of the bimetallic type in which no electrical connections are made to the bimetallic element.

It is an additional object of the present invention to provide an improved thermostat of a bimetallic type in which the response of the thermostat may be calibrated by a mechanical means and the threshold temperature may be set by an electrical adjustment means.

Objects and advantages of the invention will be apparent to those skilled in the art upon consideration of the invention as disclosed herein.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic illustration of one embodiment of the present invention;

FIG. 2 is a partial side view showing one arrangement of the permanent magnet and the saturable magnetic core;

FIG. 3 is a schematic illustration of an alternate embodiment of the present invention; and

FIG. 4 is a partial side view showing an alternate arrangement of the permanent magnet and the saturable magnetic core.

TECHNICAL DESCRIPTION OF THE INVENTION FIG. I shows one embodiment of the present invention in which a J-shaped bimetallic strip is secured to a fixed wall or housing (not shown) so that it may rotate about a pivot pin 12 that is located at the curved portion of the strip 10. The position of the. bimetallic strip 10 may be controlled by a conventional adjusting 20, which is mounted on a separate printed circuit board 18 which is secured to a fixed wall or support (not shown). The magnetic core 20 is preferably of a torroidal shape although it may be formed of other shapes if desired. When the environmental temperature is relatively low, the bimetallic arm 10b of the embodiment of FIG. 1 will be positioned so that the permanent magnet 16 or 16' is sufficiently removed from the magnetic core 20 that the core 20 will be relatively magnetically unsaturated.

The magnetic core 20 is wound with a primary wind ing 22 and a secondary winding 24, which along with the other electrical circuits of the thermostat are mounted on the board 18. The primary winding 22 is supplied with time-varying signals, preferably in the form of pulses by a conventional driver circuit 26. Thus, when themagnetic core 20 is relatively magnetically unsaturated, the pulses supplied to the primary winding 22 will produce relatively large voltage output pulses in the secondary winding 24. The output of the secondary winding 24 is coupled to a Schmitt trigger circuit 28 on the board 18. The output of the Schmitt trigger circuit 28 may be coupled to the input of a conventional retriggerable multivibrator 30 to provide the output signal, if desired. The multivibrator 30 supplies an output signal across the output terminals 32 and 34.

The threshold level of the Schmitt trigger circuit 28 is controlled by the variable resistor 36 which is connected to the resistor'38. Calibration of the thermostat could be achieved by'changing the value of resistor 38. It also may be achieved by adjusting the knob 14. The temperature level of the thermostat of the present invention is, thus, controlled by adjusting the threshold level setting of the resistor 36, which forms part of a conventional bias network for the Schmitt trigger circuit 28. The power supply 40 supplies voltage to all of the circuits on the board 18, including the driver 26, the Schmitt trigger circuit 28 and the multivibrator 30.

When thetemperature of the surrounding environment increases, expansion of the bimetallic elements 10a, IObof the bimetallic strip 10 of FIG. 1 occurs at different rates and the bimetallic arm 10b will, therefore, deflect so that the leg 10b moves toward the saturable magnetic core 20. As the permanent magnet 16 of FIG. 2 is inserted more and more into the central aperture 23 of the magnetic core 20 or the U-shaped permanent magnet 16 of FIG. 4 encloses more and more of magnetic core 20, the core 20 will become progressively more magnetically saturated and the output pulses to the Schmitt trigger circuit 28 becomes lower and lower in magnitude. An output pulse will not be produced by the Schmitt trigger circuit 28 when the magnetic saturation of the core 20 reaches a point that the voltage of the output pulses is not sufficient to maintain the actuation of the Schmitt trigger circuit 28. This point is determined by the setting of the variable resistor 36. When its temperature changes in the opposite direction and the bimetallic strip 10 points so as to displace the magnet 16 or 16' away from the core 20. In this event, the saturation of the core 20 decreases sufficiently at some point so that the output pulses from the winding 24 again actuate the Schmitt trigger circuit 28. The linear portion of the magnetization curve of core 20 is preferably utilized in the operation of the thermostat of the present invention. The Schmitt trigger circuit 28, thus, provides a signal that is representative of a predetermined threshold temperature. The

3 magnetic attraction between the permanent magnet 16 or 16' and the magnetic core 20 and the inherent electrical hysteresis of the'Schmitt trigger circuit prevents the unit from cycling on and off when small temperature changes around the threshold temperature occur.

The embodiment of FIG. 3 operates in a manner similar. to the operation of the embodiment of FIG. 1. Like the embodiment of FIG. 1, the embodiment of FIG. 3 may employ either the permanent magnet 16 of FIG. 2 or the permanent magnet 16 of FIG. 4, or a magnet of some other suitable shape. The main difference in the two embodiments being that the bimetallic strip is positioned below and to the right of the magnetic core in FIG. 3. Therefore, when the environmental temperature is'low, the leg 10b of the strip 10' of FIG. 3 will be deflected to the left and the associated permanent magnet will, thus, tend to'saturate the magnetic core 20. As the temperature of the environment'increases, the bimetallic strip 10' will be deflected to the right and the permanent magnet 16 or 16' will be increasingly removed from the magnetic core 20 so that it tends to magnetically unsatu'rate the core 20'. When the core 20 is relatively magnetically unsaturated, pulses from the driver 26 will be coupled from the primary winding 22' to the secondary winding 24 at a relatively high voltage level, and when the core 20 is relatively magnetically saturated, pulses on the secondary, winding 24' will have a relatively low voltage level.

Output pulses from the secondary winding 24 are fed to the Schmitt trigger circuit 28. The output of the Schmitt trigger circuit 28' may be coupled to the input ofa retriggerable multivibrator 30. The threshold of the Schmitt trigger circuit 28 and hence the threshold of the thermostat can be controlled by variable resistor 36'. The output of the multivibrator is taken across the output terminals 32' and 34'. The power supply 40 supplies voltage to the driver 26' and multivibrator 30. Mechanical calibration adjustment of the thermostat of FIG. 3 may be achieved by means of the cam 15 and the adjustment knob 14' so as to adjust the position to the bimetallic strip 10 about the pivot pin 12 while the threshold operating temperature of the thermostat is determined by the setting of the resistor 36'.

While particular embodiments of the present invention have been described herein, other embodiments within the scope of the present invention will be apparent to those skilled in the art.

What is claimed is:

1. An electrical thermostat comprising a saturable magnetic element, a bimetallic strip movable with respect to said magnetic element in response to environmental temperature. a permanent magnet affixed to said bimetallic strip which is movable to a first position in close magnetic coupling with said magnetic element so that said magnetic element will be relatively magnetically saturated when the environmental temperature is at a first value and is movable to a second position wherein said magnetic element will be relatively magnetically unsaturated and the environmental temperameans for supplying electrical time-varying signals to said primary winding and output means coupled to said secondary winding for receiving time-varying output voltage signals the magnitude of which varies inversely with the degree of magnetic saturation of said magnetic element.

2. A thermostat as claimed in claim 1 wherein said saturable magnetic element is a magnetic core of a torroidal shape.

3. A thermostat as claimed in claim 1 wherein said time-varying output voltage signals are pulse signals.

4. A thermostat as claimed in claim 1 comprising me-v chanical calibration means for adjusting the orientation of said bimetallic strip and electrical adjustment means I roidal shape.

7. An electrical thermostat comprising a saturable magnetic element, a bimetallicstrip movable with respect to said magnetic element in response to environmental temperature, a permanent magnet affixed to said bimetallic strip which is movable to a first position in close magnetic coupling with said magnetic element so that said magnetic element will be relatively magnetically saturated when the environmental temperature is at a first value and is movable to a second position wherein said magnetic element will be relatively magnetically unsaturated and the environmental temperature is at a second value, a primary winding and a secondary winding associated with said magnetic element, means for supplying electrical time-varying signals to said primary winding and a Schmitt trigger circuit coupled to said secondary winding for receiving timevarying output voltage signals the magnitude of which varies inversely with the degree of magnetic saturation of said magnetic element.

8. A thermostat as claimed in claim 7 wherein said saturable magnetic element is a magnetic core of a torroidal shape.

9. A thermostat as claimed in claim 7 wherein said time-varying output voltage signals are pulse signals.

10. A thermostat as claimed in claim 7 comprising mechanical calibration means for adjusting the orientation of said bemetallic strip and electrical adjustment means coupled to said Schmitt trigger circuit for adjusting the temperature response of said thermostat.

11. A thermostat as claimed in claim 10 wherein said time-varying output voltage signals are pulse signals.

12. A thermostat as claimed in claim 11 wherein said saturable magnetic element is a magnetic core of a torroidal shape. 

1. An electrical thermostat comprising a saturable magnetic element, a bimetallic strip movable with respect to said magnetic element in response to environmental temperature, a permanent magnet affixed to said bimetallic strip which is movable to a first position in close magnetic coupling with said magnetic element so that said magnetic element will be relatively magnetically saturated when the environmental temperature is at a first value and is movable to a second position wherein said magnetic element will be relatively magnetically unsaturated and the environmental temperature is at a second value, a primary winding and a secondary winding associated with said magnetic element, means for supplying electrical time-varying signals to said primary winding and output means coupled to said secondary winding for receiving time-varying output voltage signals the magnitude of which varies inversely with the degree of magnetic saturation of said magnetic element.
 2. A thermostat as claimed in claim 1 wherein said saturable magnEtic element is a magnetic core of a torroidal shape.
 3. A thermostat as claimed in claim 1 wherein said time-varying output voltage signals are pulse signals.
 4. A thermostat as claimed in claim 1 comprising mechanical calibration means for adjusting the orientation of said bimetallic strip and electrical adjustment means coupled to said output means for adjusting the temperature response of said thermostat.
 5. A thermostat as claimed in claim 4 wherein said time-varying output voltage signals are pulse signals.
 6. A thermostat as claimed in claim 5 wherein said saturable magnetic element is a magnetic core of a torroidal shape.
 7. An electrical thermostat comprising a saturable magnetic element, a bimetallic strip movable with respect to said magnetic element in response to environmental temperature, a permanent magnet affixed to said bimetallic strip which is movable to a first position in close magnetic coupling with said magnetic element so that said magnetic element will be relatively magnetically saturated when the environmental temperature is at a first value and is movable to a second position wherein said magnetic element will be relatively magnetically unsaturated and the environmental temperature is at a second value, a primary winding and a secondary winding associated with said magnetic element, means for supplying electrical time-varying signals to said primary winding and a Schmitt trigger circuit coupled to said secondary winding for receiving time-varying output voltage signals the magnitude of which varies inversely with the degree of magnetic saturation of said magnetic element.
 8. A thermostat as claimed in claim 7 wherein said saturable magnetic element is a magnetic core of a torroidal shape.
 9. A thermostat as claimed in claim 7 wherein said time-varying output voltage signals are pulse signals.
 10. A thermostat as claimed in claim 7 comprising mechanical calibration means for adjusting the orientation of said bemetallic strip and electrical adjustment means coupled to said Schmitt trigger circuit for adjusting the temperature response of said thermostat.
 11. A thermostat as claimed in claim 10 wherein said time-varying output voltage signals are pulse signals.
 12. A thermostat as claimed in claim 11 wherein said saturable magnetic element is a magnetic core of a torroidal shape. 